1. Field of the Invention
This invention relates to a plasticizing and injecting unit for use in an injection molding machine, comprising a carrier and supply block, a plasticizing cylinder detachably mounted in the carrier and supply block, which is formed with at least one tapped bore that is parallel to the injection axis, and at least one cylindrical hydraulic actuator, which is received in said bore and has external screw threads engaging the internal screw threads of said bore, wherein an angular misalignment may occur between the axis of the actuator and the axis of the bore under the influence of a misaligning moment and means are provided for locking the actuator unit in the bore and comprise at least one locking screw for weding the internal and external screw threads against each other to lock the actuator in the bore.
2. Description of the Prior Art
A known plasticizing and injecting unit of that kind is disclosed in U.S. Pat. No. 4,850,841. In that known unit a vertical slot is formed in the carrier and supply block between the bores of any pair of bores disposed one over the other and the width of that slot can slightly be decreased by means of a horizontal clamp screw so that the carrier and supply block is reversibly deformed adjacent to the slot. For this reason the hydraulic actuators received in the bores can axially be fixed in that the screw threads are wedged against each other. But a relatively heavy implement is required to tighten the clamp screw. Before that fixation the cylinders of the hydraulic actuators are usually in a position which differs from a coaxial position on the center lines of the associated bores but slightly deviate from such coaxial position. Said deviations will remain between the limits which are defined by the backlash between the internal screw threads of the bores and the external screw threads of the cylinders and are also caused by relatively strong misaligning moments, which are due to the fact that the internal and external screw threads establish a connection between units which have centers of gravity which are disposed in front of and behind the screw threads, respectively, at a considerable distance from the screw threads. A subassembly consisting of the carrier and supply block and the plasticizing cylinder is supported by the internal screw threads of the bores on the external screw threads of the cylinders of the advance-retract actuators for axially displacing the entire injection unit. Another subassembly consists of the injection actuators, a feed screw actuating bridge, which connects said injection actuators on the rear and is provided with a rotary drive, and the feed screw and is virtually suspended from the feed screw actuating bridge by the engagement between the external screw threads of the cylinders of the injection actuators and the internal screw threads of the carrier and supply block. In dependence on the design of the plasticizing and injecting unit the center of gravity of each of said subassemblies may be disposed at different locations so that misaligning moments may arise which in different plasticizing and injecting units may differ in magnitude and also in direction. In any case any deviation from a coaxial arrangement which is present before the screw threads are wedged against each other will be preserved when the screw threads are locked by being wedged against each other. It must be pointed out that even slight deviations of the hydraulic advance-retract actuators for the entire plasticizing and injecting unit from a coaxial arrangement may give rise to considerable stresses in the system owing to the large length of the plasticizing cylinder. Stresses may also be caused by a deviation of the hydraulic injection actuators for axially actuating the feed screw, which latter actuators are fixedly connected to the feed screw actuating bridge and via the bridge to the rotary drive motor for the feed screw. In that case too, even slight deviations of the hydraulic injection actuators for axially actuating the feed screw will tend to change the position of the feed screw at its forward end. Such stresses will arise even if the plasticizing cylinder and/or the carrier and supply block are supported by rolling elements on a pedestal of the injection molding machine and in that case will substantially be effective in the region between the region in which the screw threads engage each other and the means for supporting and guiding the plasticizing cylinder and the carrier and supply block.
On the other hand, the above-mentioned deviations from a coaxial arrangement may be neglected because they virtually do not become apparent during a normal injection molding operation and will not produce substantial results if the plasticizing cylinder is supported and is laterally guided. The direction of the axis of the feed screw is substantially determined by the mounting of the feed screw in the plasticizing cylinder so that the feed screw will not be bent. For these reasons the above-mentioned deviations from a coaxial arrangement will give rise in the overall system of the plasticizing and injecting unit only to stresses which are so small that they are usually not detected. But long-term trial operations have revealed that such stresses do influence the life of the operating units and members concerned and particularly of the feed screw. Even a very slight bending occurring in the plasticizing cylinder as a result of a deviation from a coaxial arrangement will result during the rotation of the feed screw to a continual mechanical stressing of the material. Besides, the surfaces of feed screws are often finished to have a brittle consistency so that the durability of such brittle surfaces may adversely be affected considerably by such stresses.
The problem residing in the deviation from a coaxial arrangement will arise wherever a subassembly having external screw threads is received in internal screw threads of a carrying structure and a strong misaligning moment arises.